The present invention relates to a method of stabilizing the operation of a hybrid spark chamber by adding a high speed discharge circuit to a proportional chamber portion of the hybrid spark chamber used as a radiation image detector such as a .beta.-ray camera or the like, operating the high speed discharge circuit every time when a spark is generated in the spark chamber portion so as to instantaneously decrease an electric field intensity of the proportional chamber portion, and preventing the proportional chamber portion from generating any induced spark, so as to prevent generation of spurious sparks independently of radiation.
Recently, in the fields of biochemistry, pharmacology or the like, together with development of autoradiography, radiochromatography or the like, in order to measure distribution of radioactive isotopes separated on a plane in a short time, use has been made of a .beta.-ray camera for carrying out an imaging by detecting the position of charged particle radiation (mainly .beta.-rays) radiated from a sample.
As such a radiation position detector, there is a self-trigger type spark chamber, which is considered to be most excellent in detectors available at present in convenience, high resolution, high detection efficiency to charged particle radiation such as .beta.-rays or the like. However, a spark chamber having a usual two-electrode structure easily generates spurious sparks independently of radiation and has difficulty in obtaining stability of the necessary action in practice. As a detector for solving such difficulty, there has been proposed a spark chamber having a hybrid structure by adding a parallel and flat plate type proportional chamber in front of the common parallel and flat plate type two-electrode spark chamber so as to operate the spark chamber with lower electric field intensity than usual (Japanese Patent Laid-open No. 30,380/78). This hybrid spark chamber has convenience, high detection efficiency and high resolution, and improves non-uniform sensitivity and instability of the operation, but has such a drawback that a spark is induced in the proportional chamber portion by the generation of a precedent spark in the spark chamber portion.
In such a conventional hybrid spark chamber, in order to obtain good stability, it is necessary to avoid fine dust or organic contamination deposited on the cathode surface of the proportional chamber portion, but in the case of using it as a .beta.-ray camera, when the cathode of the proportional chamber portion is made to contact with a sample to be measured without providing any window, it is impossible to avoid any deposit of fine dust on the cathode surface, and with the existence of an induced spark, an organic substance contained in a working gas is burnt on the cathode surface, so that it is difficult to maintain the clean surface state.
If such induced spark is generated, an electron emission source for causing generation of a spurious spark is locally formed on the cathode of the proportional chamber portion, so that instability of the operation caused by the hybrid spark chamber cannot completely be eliminated.